The invention relates to a tensioner with a damping mechanism and a belt drive system that includes such a tensioner. More particularly, the invention relates to a tensioner with a torsional spring that biases the position of a pivot-arm to which a belt engaging pulley is rotatably mounted. The tensioner of the invention with its damping mechanism is particularly useful in controlling tension of a V-ribbed belt drive system such as a front end accessory drive for automotive engine applications.
A mechanical tensioner is used to automatically control the tension of a V-ribbed belt of a front end accessory drive for automotive engine applications. Such a tensioner has a pivot-arm that rotates about a pivot secured to a base and uses a sleeve-bushing on the pivot to provide a bearing surface for the rotating pivot-arm. Many of such bushings are made of plastic and are subject to wear over the expected life of the tensioner. A torsional spring is often used with one end connected to the pivot-arm and the other end interconnected through the base to bias the position of the pivot-arm and position an attached pulley against a belt. The spring is also used to generate a spring force operative with a damping means that generates a normal force component to a friction sliding surface to inhibit or dampen oscillatory movements of the pivot-arm.
In some tensioner designs, the bearing for the pulley is in the same radial plane as the bushing so that bushing loads are minimized and bushing wear does not appreciably lead to "free play" or "cocking" of the pivot-arm with attendant misalignment of the pulley over the expected operative life of the tensioner. Examples of belt tensioners with the pulley bearing aligned in a common radial plane with the pivot-arm pivot bushing are disclosed in U.S. Pat. Nos. 4,696,663 and 5,045,031.
Another belt tensioner design centralizes the location of the pivot-arm between oppositely wound torsional springs and locates the pulley bearing midway between nylon insert bushings. The symmetrical design of the springs and location of the bushings result in substantial equal wear of each bushing. An example of such a tensioner is disclosed at Page 8 of SAE Technical Paper Series Number 790699. Problems of such tensioners include: their bulk because of the two spring design making them unsuitable for fitting within the available space limitations; their cost because of the several number of parts associated with the two spring design; and their lack of a sophisticated damping mechanism.
A tensioner design that solves the bulk, cost, and damping problems of the aforementioned SAE tensioner is disclosed in U.S. Pat. No. 4,473,362. The '362 tensioner has a pivot-arm attached to an off-set cylindrical member that supports the pivot-arm and rotates about a pivot secured to a base. Only one torsional spring is used with one end connected to the pivot-arm and the other end connected to the base. A single sleeve-type bushing on the pivot has a bearing surface that supports the cylindrical member. The radial plane of a pulley bearing is off-set in relation to the sleeve-type bushing which introduces a moment or couple as a load which must be carried by the bushing. Such tensioners are sometimes referred as "Zed" type tensioners because of the off-set of the pulley relative to its support or base structure. Unequal pressure loads introduced into bearing surfaces of the bushing can result in excessive bushing wear and attendant pulley misalignment.
The belt of a drive system utilizing such Zed type tensioner engages the pulley and generates a belt force at the pulley which is transmitted to the cylindrical member (hereinafter hub load). As explained in the '362 patent, the unequal loads to the bushing are reduced by a damping means that generates a normal force component acting in generally the same direction as the belt force component transmitted to the cylindrical member. Although the orientation of the belt force with the force component certainly alleviates some of the bushing load and attendant wear problems, it is lacking in some belt drive situations because the normal force component of the damping means is insufficient to balance against a moment generated by the belt force being off-set from the cylindrical member carrying the hub load, and the single bushing has a tendency to "bevel" or "crown" as bearing pressure loads change as the pivot-arm oscillates between clockwise and counter clockwise directions. In one direction of pivot-arm rotation, the friction force generated by the damping mechanism is additive to the loads to the bushing while in the counter direction of pivot-arm movement, the friction loads subtract. Eventually, "beveling" or "crowning" of the bushing due to unequal wear permits the cylindrical member and attached pivot-arm to "rock" causing misalignment of the pulley relative to the belt of a belt drive system. As manufactured, such tensioners may have pulleys aligned to plus or minus 0.5.degree. but after extended use, the beveling of the bushing causes pulley misalignment as high as plus or minus 1.5.degree..